1. Field of the Invention
The present invention relates to a thread cutting device for a sewing machine. More particularly, the invention relates to an automatic thread cutting device for a sewing machine incorporating an improved movable blade unit capable of catching both the needle thread and the bobbin thread when cutting the threads and capable of cutting the threads in cooperation with a fixed blade.
2. Description of the Related Art
A known automatic thread cutting device for a sewing machine is disposed between the throat plate and the loop catcher and is driven for timed action by the rotating hook shaft to cut both the needle thread and the bobbin thread automatically.
A thread cutting device proposed in Japanese Utility Model Laid-open (Kokai) No. Hei 2-148675 is provided with a movable blade unit comprising an upper and a lower plate, and an arresting plate formed of a synthetic resin connected to the tips of the upper and lower plates. The arresting plate has an arresting portion for arresting the needle thread and the bobbin thread. The upper plate is provided with a movable blade, and the lower plate has a thread drawing portion for drawing the threads during cutting. The movable blade unit is reciprocated by a driving mechanism at the end of the sewing operation to arrest the needle thread and the bobbin thread. Then, as the needle thread and the bobbin thread are drawn by the thread drawing portion, the needle thread and the bobbin thread are sheared off between the movable blade and a fixed blade.
This known thread cutting device with a movable blade unit comprising upper and lower plates and an arresting member is unable to cut the threads if the shape of the junction of the arresting member and the upper and lower plates is not smooth. When the junction is rough, the threads are caught by the junction when the movable blade unit is reciprocated.
To solve this problem, the inventors of the thread cutting device disclosed in Japanese Utility Model Laid-open (Kokai) No. Hei 2-148675 formed the movable blade unit only by upper and lower plates as disclosed in Japanese Patent Laid-open No. 5-49778. An arresting portion is formed in the lower plate, and a guide portion is formed in the free end of the lower plate to prevent the threads from entering the space between the upper and lower plates. The guide portion is formed by bending up the free end of the lower plate so that the extremity of the free end protrudes slightly from the upper plate. Such improvements prevent the movable blade unit from catching the needle thread and the bobbin thread when cutting the threads and enhance the reliability of the thread cutting device.
However, since the lower plate of the movable blade unit has the guide portion formed by bending its free end upward in addition to the thread drawing portion and the arresting portion, the lower plate has a complex shape. Thus, the lower plate requires plural processes to form the plate, which increases the cost of the movable blade unit.